Changing node that effects to all material
Rather than using a Color Ramp, you can create a Node Group to hold your colours as separate RGB input nodes. Your materials can then use the same Node Group to make use of those colours and changing the contents of the Node Group will affect everything that uses it.
To create your Node Group, first create all of your RGB nodes and highlight them all (by dragging a box around them). In the Add menu there's an option for 'New Group' (within the Group sub-menu). This will create your node group.

You can rename the output sockets of the group using the properties panel (N).
You can then add your group (Add/Group/NodeGroup) to your materials :

Note that you can rename the node group by clicking in the Name field (it defaults to 'NodeGroup' but you can give it any name you like - such as 'Color Palette').
Making changes to the internal nodes of the group will affect anywhere that uses that group.